The output of a semiconductor laser will vary over time as the characteristics of the device change, and may also vary in response to changes in temperature, etc. It is important for lasers used in optical communications systems that the laser output is stable with regard to the power and the wavelength of the transmitted light. Known systems that are used for monitoring the wavelength and power level of use interferometers, such as, for example, a Mach-Zehnder interferometer, and consequently are expensive and complex.